1. Field of the Invention
The present invention relates to thermoplastic compositions utilizing polymers of monovinylaromatic compounds which have been modified with rubber to increase their impact strength. More particularly the present invention relates to the production of high impact polystyrene (HIPS) predominantly containing graft copolymer particles having a core-shell structure with a polystyrene core occluded inside a polybutadiene shell. The predominant core-shell morphology of the invention is preferably achieved through the use of toluene as a solvent in the batch polymerization of styrene in the presence of polybutadiene.
2. Description of Related Art
Three terms commonly used by those of skill in the art to describe the morphologies of graft copolymer particles in HIPS are core-shell, cell and maze. The correlation between improved gloss in HIPS and the number of graft copolymer particles exhibiting a core-shell morphology is also well known. A considerable amount of grafting is required in order to produce the core-shell morphology. The conventional method for achieving a higher concentration of core-shell graft structures in HIPS is through the use of SBR (styrene-butadiene rubber) block copolymers, or SBS or BSB terpolymers, as the rubbery components. In the case of styrene-butadiene copolymers it is common to use styrene-butadiene diblock polymers that contain 60 to 80 percent polybutadiene blocks and 20 to 40 percent polystyrene blocks.
Although economics favor the use of polybutadiene instead of SBR as the rubbery component in HIPS, conventional batch polymerizations of styrene using polybutadiene in hexane or cyclohexane solvents tend to produce cell structures as the predominant morphology. Using ethylbenzene as the solvent and polybutadiene as the rubber component in the production of HIPS produces significant quantities of all three morphologies. These results are unsatisfactory where the HIPS is intended for use in applications requiring higher gloss.
U.S. Pat. No. 4,528,327 discloses thermoplastic compositions comprising a polyphenylene ether and a rubber modified polystyrene resin containing a majority of core-shell structures having a polystyrene core inside a shell of polybutadiene.
Japanese Patent 2038435 discloses a rubber-modified styrene composition wherein the rubbery polymer is present in both core-shell and cell morphologies. Disclosed core-shell structures include styrene type polymer cores within rubbery polymer shells made of natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene copolymer rubber, styrene-isoprene copolymer rubber, butyl rubber, and ethylene-propylene rubber.
Japanese Patent 2057311 discloses a rubber-modified polystyrene resin for use in making injection blow molded products. The resin comprises a polystyrene continuous phase and dispersed rubber particles having a core-shell structure in which polystyrene cores are enveloped with shells of rubber films. The average particle size ranges from 0.3 to 2.0 microns, and the resin has a graft rubber content of 1-15 weight percent. The rubbers disclosed for use in the invention include polybutadiene, polybutene and polyisoprene. The rubber particles are said to give high transparency, surface gloss and strength to the molded products.
The invention disclosed herein is a rubber-modified thermoplastic composition made by polymerizing a monovinyl aromatic polymer in the presence of rubber and a solvent to produce a resin having a core-shell morphology. According to one preferred embodiment of the invention, HIPS having a predominant core-shell morphology is made by polymerizing styrene in the presence of polybutadiene using toluene as a solvent. The thermoplastic polymer composition of the invention is characterized by a continuous phase of polystyrene containing dispersed graft copolymer particles having a core-shell structure with a polystyrene core occluded inside a polybutadiene shell.
A particularly preferred HIPS of the invention is produced when the styrene is batch polymerized using styrene/toluene mixtures of about 70:30 weight percent ratios in the presence of from about 9 to about 15 weight percent polybutadiene, while flashing off the toluene solvent. The solubilized polybutadiene is preferably either high or medium cis, and the peroxide initiator level preferably ranges from about 250 to about 500 ppm. A preferred temperature profile for the polymerization is about 110xc2x0 C. for about 75 minutes, followed by about 130xc2x0 C. for about 60 minutes and about 150xc2x0 C. for about 60 minutes.